Image forming apparatuses, such as copying machines and MFPs (Multi Function Peripherals) which are equipped with a color printing function, are now popularly used. In the case where color printing is conducted by using such an image forming apparatus, it is important to perform color administration in order to reproduce correctly colors contained in each page of a job. Then, the conventional color administration has been performed in the following ways. An inline sensor disposed in an image forming apparatus is configured to measure a color patch arranged on an end of a printed matter (paper sheet), and a controller to control the image forming apparatus is configured to compare color information extracted from a job with color information read with the inline sensor so as to acquire an amount of a change of each color, and to perform feedback control of colors in real time so as to make the amount of the change of each color fall in a proper range.
With regard to such the feedback control, Japanese Unexamined Patent Publication (JP-A) No. 2005-181673 discloses an electro-photographic color image forming apparatus which superimposes multi-colored toner images on a transfer sheet so as to form a full color image. This color image forming apparatus includes a color optical density sensor disposed so as to measure an after-fixing image density (i.e., the image density of an image after fixing) and is configured to perform feedback control so as to minify a difference between the write-in data of a measurement target image and the measurement result of the target image by the color optical density sensor. The color image forming apparatus is further configured to calculate an amount of a shrink of an image caused at the time of fixing processing based on the relationship between the position information of an after-fixing image and the position information of a before-fixing write-in image, to calculate an after-fixing position of a measurement target image to be used to measure the image density of an after-fixing image based on the above calculation result, and to measure the image density of the measurement target image based on the above calculation result.
The functionality of RIP (Raster Image Processor) processing to rasterize the data of a job so as to produce image data for each page has been made higher from year to year. In particular, in color printing, it becomes possible to conduct highly-skilled setting in image forming apparatuses so as to perform color administration by setting a tone curve and a color profile for each kind of objects and each color space.
However, in order to perform feedback control for each kind of objects and each color space, it is necessary to arrange a lot of color patches prepared for each kind of objects and each color space at an end portion of a printed sheet. Accordingly, it is difficult to secure a space to arrange a lot of color patches in a printed matter. Further, if colors to form the color patches are limited in order to minify the space, color patches for colors contained mainly in a printed matter can be dropped, which causes the case that appropriate color administration cannot be performed. The present invention seeks to solve the problem.